Electrical fuse

ABSTRACT

A multi-element high-voltage fuse comprises a plurality of similar fuse elements connected in parallel, each element being a strip of fusible material having a plurality of first portions of reduced cross-section spaced along its length and, between each mutually adjacent pair of said first portions, a plurality of shorter second portions of reduced cross-section. Under shortcircuit conditions, substantially all the first and second portions of all the elements fuse simultaneously. Under low overload conditions, the elements fuse one at a time, at their longer first portions only, but the conditions are such that multiple arcing occurs, at substantially all these longer first portions, leading to safe fuse operation, at lower values of overcurrent than in conventional designs; and since the longer first portions are much less numerous than the shorter second portions they have relatively little effect on the overall resistance of the fuse and thus on its current carrying capacity. Conversely, fewer parallel elements are required to achieve a given minimum breaking current, since a lower current density per element suffices to produce multiple arcing. Thus, for a given minimum breaking current, the fuse may be more compact and robust than hitherto. In general, each element should contain about one of the longer second portions per kilovolt of the voltage rating of the fuse, for safe operation of the fuse under low overload conditions.

United States Patent Rosen et a1.

ELECTRICAL FUSE Inventors: Philip Rosen; John Feenan, both of Liverpool,England; Kenneth Douglas Howton, Pallavaram, Madras, India [73]Assignee: The English Electric Company Limited, London, England Filed:Mar. 20, 1972 Appl. No.: 236,018

Related US. Application Data Continuation-in-part of Ser. No. 74,429,Sept. 22, 1970, abandoned.

[30] Foreign Application Priority Data Sept. 23, 1969 Great Britain46794/69 [52] US. Cl 337/161, 337/159, 337/160, 337/290, 337/293,337/295 Int. Cl. H0lh 85/12 Field of Search 337/158, 159, 161, 240,337/293, 295, 296, 297, 163

[56] References Cited UNITED STATES PATENTS FOREIGN PATENTS ORAPPLICATIONS 1,538,414 9/1969 Germany 337/161 Sept. 10, 1974 PrimaryExaminerJ. D. Miller Assistant Examiner-Fred E. Bell Attorney, Agent, orFirmMisegades, Douglas & Levy [57] ABSTRACT I A multi-elementhigh-voltage fuse comprises a plurality of similar fuse elementsconnected in parallel, each element being a strip of fusible materialhaving a plurality of first portions of reduced cross-section spacedalong its length and, between each mutually adjacent pair of said firstportions, a plurality of shorter second portions of reducedcross-section. Under short-circuit conditions, substantially all thefirst and second portions of all the elements fuse simultaneously. Underlow overload conditions, the elements fuse one at a time, at theirlonger first portions only, but the conditions are such that multiplearcing occurs, at substantially all these longer first portions, leadingto safe fuse operation, at lower values of overcurrent than inconventional designs; and since the longer first portions are much lessnumerous than the shorter second portions they have relatively littleeffect on the overall resistance of the fuse and thus on its currentcarrying capacity. Conversely, fewer parallel elements are required toachieve a given minimum breaking current, since a lower current densityper element suffices to produce multiple arcing. Thus, for a givenminimum breaking current, the fuse may be more compact and robust thanhitherto. In general, each element should contain about one of thelonger second portions per kilovolt of the voltage rating of the fuse,for safe operation of the fuse under low overload conditions.

10 Claims, 3 Drawing Figures ELECTRICAL FUSE CROSS-REFERENCES TOPERTINENT INFORMATION AND REFERENCES This is a continuation-in-partapplication of Ser. No. 74,429, filed Sept. 22, 1970 now abandoned.Applicants claim the benefit of the right of priority under 35 U.S.C.119, based on British application No. 46794/69, filed Sept. 23, 1969.Prior art references of cursory value are:

2,181,825 Wood 2,833,891 Jacobs 2,489,501 Rensner 3,287,525 Mikulccky2,667,549 Fannoe 3,611,239 Kozacka 2,670,418 Kozacka 51742 Romania,patent 2,770,033 Kozacka 1,325,090 France. patent 1,538,414 Germany,patent This invention relates to electrical fuses and more particularlyrelates to high voltage fuses, of high rupturing capacity, designed forboth low overcurrent and short-circuit duties.

Hitherto, high voltage fuses designed to give a good low overcurrentperformance have principally employed either a large number of regularlynotched strip elements, connected in parallel, or a small number ofparallel-connected compound elements each made up from sections ofsilver strip and silver wire in series, the wire sections being bestable to deal with overcurrent faults whilst the strip section, suitablynotched, is best able to deal with short-circuit duties.

Such fuses however suffer from numerous disadvantages, the former typeinvolving the danger of fracture in service by reason of the necessarilysmall crosssection of the notched necks in the large number of elementsemployed whereas the other type is difficult to fabricate and possessesa high electrical resistance re-' sulting in an inferior currentcarrying capability for given body dimensions. Further, the latter typemay afford a high energy let-through under short-circuit conditions.

Accordingly, it is an object of this invention to provide an improvedfuse which obviates or at least largely mitigates these disadvantages.

According to the present invention, there is provided a multi-elementhigh-voltage fuse comprising a plurality of similar parallel-connectedfuse elements, wherein each fuse element comprises a fusible striphaving a plurality of first portions of reduced cross-section spacedapart along the strip and, between each mutually adjacent pair of saidfirst portions, a plurality of spaced-apart second portions of reducedcross-section, each of said second portions being shorter than each ofsaid first portions.

In such a fuse according to the invention, each fuse element may includea plurality of first notches equidistantly spaced apart along the stripand forming the said first portions of reduced cross-section, and,between each mutually adjacent pair of said first portions so formed, aplurality of shorter second notches equidistantly spaced apart along thestrip and forming said second portions of reduced cross-section.

In order that the invention may be fully understood, one embodimentthereof will now be described with reference to the accompanyingdrawing, in which:

FIG. 1 is an elevational view of the major parts of a typical fuseaccording to the invention,

FIG. 2 illustrates the construction of a fuse element comprised by thefuse shown in FIG. 1,

FIG. 3 is a graphical illustration of critical current density againstnotch length, with reference to a fuse element as shown in FIG. 2.

The fuse shown in FIG. 1 comprises an elongate electrically-insulatingand temperature-resistant former l1 fitted with electrically conductiveend caps 12 and 13. The former 11 has a plurality of equiangularlyspaced radially projecting longitudinal ribs 14, and round it aplurality of .fuse elements 15 are wound in generally helical form, thefuse elements 15 being thus supported on the tips of the ribs 14 andotherwise out of contact with the former 11. The two ends of each fuseelement 15 are secured mechanically to the end caps 12 and 13respectively, in electrical contact therewith. Apart from the details ofthe fuse elements 15, described below, this is, of course, a well knowntype of fuse construction and, as is also well known, the former 11would in practice be surrounded by an outer fuse barrel (not shown)sealed to the end caps 12 and 13 and containing, in the enclosed spacesurrounding the former 11, a suitable arc-quenching material such assilica sand through which the fuse elements 15 extend.

Each fuse element 15, of which only the general disposition is indicatedin FIG. 1, is constructed as shown in greater detail in FIG. 2. Eachsuch element consists of a strip of electrically conductive fusiblematerial, preferably silver, having its edges notched to provide aplurality of relatively long necks 16 of restricted crosssection and,between each mutually adjacent pair of the necks 16, a plurality ofrelatively short necks 17 of restricted cross-section. Thus each fuseelement 15 has a greater number of the shorter necks 17 than it has ofthe longer necks 16.

In a fuse as described above and rated for use in a 10,000 volt supplycircuit in which, in short circuit conditions, it may have to respond toa short-circuit current of 10,000 amps or more, each fuse element 15 mayrequire to have some 30 or 40 of the shorter necks 17 whereas, asexplained below, it need have only about 10 of the longer notches 16.For the same conditions of use, and to provide a satisfactory fusingresponse to a prolonged low overload (say, in the region of two to threetimes rated current), each fuse element 15 may be of silver strip havinga width of 0.1 inches and a thickness of 0.0025 inches, each of thelonger or shorter necks 16 or 17 may have a length of 0.2 inches or 0.02inches respectively, each of the necks being about 0.025 inches in widthand adjacent necks being about 0.5 inches apart. In general, the longernecks 16 may be between four and 12 times longer than the shorter necks17, the distance between adjacent necks may be between two and fourtimes the length of the longer necks, and the width of any of the necksmay be between one-sixth and one-half that of the strip in which theyare formed.

When a fuse according to the invention, and as above described, issubjected to short circuit conditions, the high current density in allthe parallel-connected fuse elements 15 causes all or substantially allthe necks l6 and 17 of all the elements 15 to fuse substantiallysimultaneously; and, momentarily, arcing occurs at all the gaps whichresult from fusing of the necks. As is well understood in the art, thenumber of necks provided in each element 15 is chosen sufficientlylarge,

having regard to the voltage rating of the fuse (i.e., the voltage inconnection with which it is to be used), that the excess of this voltageover. the total are voltage of all the arcs occurring in seriessimultaneously in any one of the elements 15 is insufficient to maintainthe stability of these arcs; and accordingly the arcing is extinguishedalmost instantaneously in all the elements 15.

As is also well understood, under conditions of low overload in whichthe overload current is less than that at which the fuse will ruptureinstantaneously, the time required for a neck to fuse is dependent notonly on the current density flowing through it but also on its length,since the temperature rise in the neck is reduced or made less rapid byheat losses to the full-width parts of the fuse element between whichthe neck extends and these heat losses are more significant for a shortneck than they are for the central portion of a longer neck. In a givenfuse,there is an inverse relation between the length of a neck and theminimum current density below'which fusing of the neck will not occurbecause, at a temperature which is still less than the fusingtemperature of the neck, the rate of heat loss from the neck will becomeequal to the rate at which heat is generated in the neck. Accordingly, arelatively low overload current, which is insufficient to cause fusingof the short necks 17, may still be sufficient to cause fusing of thelonger'n'ecks 16.

The relationship between the lengths of the necks 16 and the minimumcurrent density which will result in fusing of these necks is, not,however, the critical factor in the design of high voltage fuses withhigh rupturing capacity. In such fuses, the achievement of high rupturing capacity in respect of low overloads necessitates (a) the provisionof a plurality of the longer necks 16 spaced from one another lengthwiseof the fuse and electrically in series and (b) an arrangement wherebyfusing and consequent arcing will occur simultaneously at a substantialnumber of these necks, whereby the energy released in the fuse as itfuses in response to a low overload will be spread over the length ofthe fuse rather than highly localised as it would be if fusing andarcingwere to occur at only one or two of the necks, and whereby,furthermore, adequately rapid extinction of the arcing will be ensured.We have found that for any given type of fuse element with a pluralityof necks 16 of given length, there is a critical current density(substantially higher than the minimum current density referred toabove), below which only one or two of the necks 16 will fuse, whereascurrent densities above this critical value will result in fusing andarcing at all or substantially all the necks 16, simultaneously.Accordingly, in designing a high voltage fuse with high rupturingcapacity, for satisfactory operation in response to low overloads aswell as to short circuit conditions, it is necessary to ensure that thecurrent density to which the fuse elements will be subjected will exceedthe critical current density during operation of the fuse in response tolow overload conditions- If this is not done, the resulting highlylocalised and too-slowly extinguished are energy will be -liable toresult in explosion or mechanical fracture of the fuse.

We have found, further, that the critical current density for the necks16, as referred to above, is a function of the lengths of these necks,asshown in FIG. 3 in which the legend: Length of element reduced sectionrefers to the length of each neck 16. As shown in FIG.

3, increasing the lengths of the necks 16 has, initially, the effect ofreducing quite markedly the critical current density which is requiredto produce substantial multiple arcing and consequent safe operation ofthe fuse in response to low overload conditions, but that furtherincreases in the lengths of these necks have progressively less effecton the critical current density. Bearing in mind that each increase inthe lengths of the necks 16 is accompanied by a corresponding increasein the resistance of the elements and thus a corresponding decrease inthe current carrying capacity of the fuse 11 (unless the cross-sectionof elements 15 is increased in compensation) it will be apparent that,for

any required combination of current carrying capacity and response tolow overload currents there is an optimum length for the necks 16.

Further, we have found that at some value of breaking current, between alow overload current and a short circuit, the shorter notches 17 takeover the main duty of interruption from the longer notches 16,the'actual value of this transitional current being dependent upon theratio between the lengths of the longer and shorter necks.

Thus, having fixed the length of the longer necks 16 for optimumperformance in response to a low overload, the shorter necks 17 arepreferably, in order to minimise the resistance of these necks, made asshort as possible consistent with the need to ensure that thetransitional current will be sufficiently low (typically in the regionof 1000 amps) for the longer necks 16 to handle with safety breakingcurrents which are just below this value.

Although a large total number of the notches l6 and 17 in series isnecessary in order to achieve rapid extinction (usually within onehalf-cycle of the power supply) of the arcs which occur on fusing inresponse to a short circuit, thereby to limit severely the time duringwhich the high short circuit current can produce a high rate of energydissipation in the fuse, a comparatively small number of the necks 16 inseries suffices to produce the slower extinction rate which ispermissible of the arcs which occur at the necks 16 in response to a lowoverload. A slower arc extinction rate (say some 3-7 cycles of thesupply power) in this latter case is permissible because the currentflowing, and thus the instantaneous power dissipation within the fuseduring arcing, is much less than in the case of a short circuit.

The foregoing discussion of the required response to theoccurrence of alow overload has been presented in terms of a single fuse element formedwith a plurality of the longer necks 16. The .relevance of thisdiscussion to fuses according to the invention, which are provided notwith a single fuse element but with a plurality of such elements inparallel, will be appreciated from the following description of the wayin which the fuse described with reference to FIGS. 1 and 2 responds toa prolonged low overload current.

A prolonged low overload current of sufficient magnitude (i.e.,producing in the necks 16 of each of the parallel-connected elements 15a current density which is greater than the minimum current densityreferred to above) results, eventually, in a single one of the necks 16of one of the elements 15 fusing first, to form a single gap in thiselement. Since this element is connected in parallel with several otherelements 15 which are still intact, there is insufficient voltage acrossthe said gap to maintain an arc thereacross, and the arc is thereforeextinguished substantially instantaneously. The current in the oneelement is thus interrupted, and the current density in the necks 16 ofthe remaining elements 15 increases correspondingly This results in aneck 16 of one of these other elements 15 fusing after only a shortinterval, to form another gap in which, similarly, the arc is quicklyextinguished so that a further increase results in the current densityin the necks 16 of the elements 15 which are then still intact. Assuccessive elements 15 are thus interrupted, the current densitiesincrease correspondingly in the necks 16 of the elements 15 stillremaining intact; and by this means it is ensured that, at least in thelast element remaining intact, the current density in the necks 16 willeventually exceed the critical current density, with the result thatrupturing of this-last element remaining intact occurs when all, orsubstantially all, its necks l6 fuse simultaneously.

Now when the necks 16 of this last element 15 fuse,

there is no parallel-connected conductive path to hold down the voltageacross the fuse to a level which is too low to maintain an are (as wasthe case for, say, the first element 15 in which a single neck 16 fusedbut the arc thereacross was immediately extinguished as describedabove). However, when fusing occurs in the last remaining element 15 itoccurs simultaneously at substantially all the necks 16 of that element,so that acorresponding plurality of arcs are produced in series; and,provided the number of these arcs is sufficiently large, the total arevoltage of all the arcs in series is correspondingly large and theexcess, over this total are voltage, of the voltage driving the overloadcurrent, will be insufficient to maintain these arcs which willtherefore rapidly become extinguished. In practice, it is found thatunder low overload conditions (in which, in the element 15 which fuseslast, the arc current and thus the instantaneous power dissipation inthe fuse is much less, perhaps by a factor of 50, than under shortcircuit conditions, so that much slower arc extinction, over, say, 3-7cycles of the power supply, is permissible) the total are voltage of theplurality of arcs in series will be sufficient to result in adequatelyrapid extinguishing of these arcs if the number of arcs involved is notless than the number of kilovolts of the supply voltage (i.e., of thevoltage rating of the fuse appropriate to the situation).

Assuming, then, that the fuse is correctly rated for the situation inwhich it is being used, and that each of its elements 15 has theappropriate number of necks 16, the last of its elements 15 will fuseand arc at all its necks l6 simultaneously and all these arcs will beextinguished with adequate rapidity. As soon as the current in this lastelement 15 is thus interrupted, the full supply voltage is appliedacross each of the other elements 15, some of which have only one fusedneck 16. This results in an are being struck again across the fused neck16 of one (but only one) of these elements 15, and the relatively highcurrent density at all the other necks 16 of this one element 15 whichare still intact causes them all to fuse simultaneously with a similarlyadequate rate of extinction of all the arcs in series which result atthe fused necks 16. Then another, and subsequently each in turn (butonly one at a time) of the elements 15 in which only one or a smallnumber of the necks 16 had previously fused responds in the same way tothe voltage to which all the elements 15 As noted above, the much lowerarc currents which occur on response of the fuse to low overloadconditions means that a much-lower arc extinction rate can be permitted,consistent with safe operation of the fuse, and this in turn means thatthe number of overload" necks 16 which must be provided in any oneelement 15, in order to ensure that the corresponding number of arcs inseries will have a sufficiently high total are voltage to provide therequisite rate'of extinction of all the arcs, is subst antially lessthan the total number of short circuit" necks l7 and necks l6'which mustbe provided in each element in order to ensure the much more rapid rateof arc extinction which is required under short-circuit conditions whenthe arc currents, and the power dissipation in the fuse, are higher. Asa result of the relatively small number of overload" necks 16 with whicheach element 15 is accordingly provided, the resistance of the fuse, andhence its current carrying capacity, are comparatively unaffected bythese necks. Also, since multiple arcing at the necks 16 is achievableat relatively low current densities per element, clue to the length ofthese necks, fuses according to the invention may employ fewer (and morerobust) elements 15 for a given overload capability. It is, however, anessential feature of the invention that the fuse should comprise aplurality of elements in parallel, so that, on initial failure of eachelement in turn during subjection of the fuse to prolonged overloadconditions, the current density in those remaining intact will increaseprogressively to such a level, exceeding the critical current density,that the initial failure of the last to fail will be by simultaneousfusing of all or most of its overload necks 16.

It should be understood that, in known manner, each element 15 may beprovided at spaced points along its length with spots or blobs of soldersuch as that shown at 18 in FIG. 2, as a protection againstlong-continued overloads at a still lower level which would beinsufficient to fuse the overload necks 16. At the so-called M- effectzones so provided, the general heating of the element 15 which resultsfrom a prolonged low-level overload results in the solder blob 18melting and its material gradually migrating into the still-solid bodyof the element 15 to form with the material thereof a eutectic solidsolution of which the melting point is substantially less than that ofthe unalloyed material of the element 15, so that the element 15eventually fuses at the position of one of the blobs 18. Once thisoccurs, a sequence of events follows which is similar to that, alreadydescribed, which occurs when the fuse responds to an overload sufficientto fuse first one, and then all or most of the others, of the overloadnecks 16. The fusing of one element 15 at one of its M-effect zonesresults in current through that element being interrupted and in anincrease in current through the remaining elements 15, so that either anM-effect zone or an overload neck 16 in one of those other elements 15 7will be caused to fuse in turn, leading to a progressive build-up oravalanche effect as above described.

Although the fuse element has been described with reference to theparticular embodiment illustrated it is to be understood that variousmodifications may readily be made without departing from the scope ofthis invention. For example, the invention is not neces sarilyrestricted to the use of notches having only two different lengths sinceone or more further groups of notches may be incorporated, dimensionedfor optimum performance under conditions intermediate between a lowovercurrent and a short-circuit, the lengths of these further groupsbeing different from the other two mentioned.

It should also be understood that although the foregoing description hasillustrated the invention only in terms of fuse elements having necksformed by notching the elements to reduce the cross-section at thenecks, the invention is in general applicable also to fuses in which thefuse elements have the required portions of reduced cross-section formedin any other suit.- able manner, such as by forming apertures throughthe elements or transverse grooves across them.

We claim:

1. A multi-element high-voltage fuse comprising a plurality of similarparallel-connected fuse elements, wherein each fuse element comprises afusible strip having a plurality of first portions (16,16) of reducedcross-section spaced apart along the strip and, interposed between eachmutually adjacent pair of said first portion (16,16) a plurality ofspaced-apart second portions (17,17 ,17 of reduced cross-section each ofsaid second portions being essentially shorter in length that the lengthof each of said first portions.

2. A fuse as claimed in claim 1, wherein each fuse element comprises atleast one of said first portions per kilovolt of the voltage rating ofthe fuse.

3. A fuse as claimed in claim 1, wherein the strips ar notched to formsaid first and second portions.

4. A fuse as claimed in claim 2, wherein the strips are notched to formsaid first and second portions.

5. A fuse as claimed in claim 1, wherein the ratio of the lengths ofsaid first and second portions is in the range from 4:1 to l2:l.

6. A fuse as claimed in claim 2, wherein the ratio of the lengths ofsaid first and second portions is in the range from 4:1 to 12:1.

7. A fuse as claimed in claim 3, wherein the ratio of the lengths ofsaid first and second portions is in the range from 4:1 to 12:1.

8. A fuse as claimed in claim 4, wherein the ratio of the lengths ofsaid first and second portions is in the range from 4:1 to l2:].

9. A fuse as claimed in claim 1, wherein each element is provided atspaced points along its length with spots of solder (18) as a protectionagainst long continued overloads at a still lower level which would beinsufficient to fuse the overload first portions (16).

10. A fuse as claimed in claim 1, wherein said plurality of fuseelements are wound in generally helical form having the ends thereof inelectrical contact with end caps.

1. A multi-element high-voltage fuse comprising a plurality of similarparallel-connected fuse elements, wherein each fuse element comprises afusible strip having a plurality of first portions (16,16) of reducedcross-section spaced apart along the strip and, interposed between eachmutually adjacent pair of said first portion (16,16) a plurality ofspaced-apart second portions (17,17,17) of reduced cross-section each ofsaid second portions being essentially shorter in length that the lengthof each of said first portions.
 2. A fuse as claimed in claim 1, whereineach fuse element comprises at least one of said first portions perkilovolt of the voltage rating of the fuse.
 3. A fuse as claimed inclaim 1, wherein the strips are notched to form said first and secondportions.
 4. A fuse as claimed in claim 2, wherein the strips arenotched to form said first and second portions.
 5. A fuse as claimed inclaim 1, wherein the ratio of the lengths of said first and secondportions is in the range from 4: 1 to 12:1.
 6. A fuse as claimed inclaim 2, wherein the ratio of the lengths of said first and secondportions is in the range from 4: 1 to 12:1.
 7. A fuse as claimed inclaim 3, wherein the ratio of the lengths of said first and secondportions is in the range from 4: 1 to 12:1.
 8. A fuse as claimed inclaim 4, wherein the ratio of the lengths of said first and secondportions is in the range from 4: 1 to 12:1.
 9. A fuse as claimed inclaim 1, wherein each element is provided at spaced points along itslength with spots of solder (18) as a protection against long continuedoverloads at a still lower level which would be insufficient to fuse theoverload first portions (16).
 10. A fuse as claimed in claim 1, whereinsaid plurality of fuse elements are wound in generally helical formhaving the ends thereof in electrical contact with end caps.